I have used several plain ol' 2 GB SD cards in my hx5v and everything works fine, including the highest resolution video mode. No problem, but my 'clips' are usually no more than 10-15 seconds in length and longer ones could possibly not work as well. I suspect that Sony uses the internal memory as a buffer too, so that shorter clips like that wouldn't depend on card speed so much.

I think that Sony is just 'covering their butt' when they say which cards to use..:-)